1. Field of the Invention
The present invention is broadly concerned with subterranean tanks for receiving sewage or serving as a cistern for holding water which are rotationally molded of synthetic resin material. More particularly, the present invention is concerned with a tank of the foregoing type provided with parabolic wall construction and a portal arrangement for accomodating a uniquely and complementally configured cover or a synthetic resin riser which is constructed in such a manner that it may trimmed in multiple locations along its axial length while retaining wall strength and the ability to couple to the tank and receive a cover thereon.
2. Description of the Prior Art
As used in this application, “subterranean tank” refers to vessels for containing liquid which are intended to be substantially completed covered with earth so as to lie below grade. These subterranean tanks are commonly used in residential and light commercial construction in suburban and rural areas where the tanks are used as a septic tank for receiving sewage and wastewater, or as a cistern for holding water for agricultural, commercial or household use. These tanks have long been constructed of concrete or metal. These tanks are generally heavy and difficult to transport and install. Over time, the concrete tanks may crack or leak, and the metal tanks may corrode. Thus, attempts have been made to develop a practical and suitable subterranean tank out of synthetic resin which is durable, lightweight and economical to produce, while being able to withstand the soil pressures and overloads as well as impacts caused by machinery and the like inherent in their use. Among the various approaches taken by the prior art is that shown and described in U.S. Pat. No. 4,254,885. This patent discloses a subterranean tank having a plurality of wall corrugations and an arched top wall, and a depression for the ingress and egress of liquids.
While the construction of subterranean tanks has been advanced by this design, in many instances there has developed a need to bury the tanks significantly below grade due to ambient temperatures, the conformation of the earth surface, or other factors. In these circumstances, it is necessary to be able to gain access to the interior of the tank from the surface for such purposes as repair or pumping of the contents of the tank. Rather than specifically configure the tank to the site, a riser is often employed to compensate for the depth of the tank while permitting access to the opening through the riser. One such riser is shown, for example, in U.S. Pat. No. 6,047,724. The riser shown and described in that patent employs a corrugated wall construction and engagement structure for permitting a plurality of risers to be connected in tandem, one atop the other. In this way, the bottom portion of the riser may be connected to the tank at the opening of the latter or to similarly constructed risers therebelow, and the top portion of the riser being adapted to receive a cover or to connect to the bottom portion of a similar riser placed thereon.
While the riser shown in U.S. Pat. No. 6,047,724 is designed for use with other risers to provide a suitable depth or height adjustability, it does not admit for adjustments in height less than the axial length of the riser. That is to say, the construction of riser of the U.S. Pat. No. 6,047,724 necessitates that risers of a variety of different lengths be available on site to permit the cover to be placed at grade level as each riser is of a definite length and not capable of adaptation to the differences of depth as the tank is buried and covered with earth discoverable only after installation is complete.
Furthermore, the covers used with the tanks of this type have traditionally been of concrete or metal which are both expensive and heavy. There has thus developed a need for a cover having increased structural strength and durability, and a cover which facilitates positioning and coupling to either an opening on the tank itself or to a riser positioned thereon.